Peter Wu
EE310, Section 5
Experiment 8 – Op Amp Design Project
DESIGN PROPOSAL
Design Schematic
Design Description and Analysis Equations
The concept of the design of the circuit is to get rid of vp’s contribution at the output vO3, scale the
voltage signal to a 2Vpeak amplitude so that it meets the change in vO3 / change in current I = 200 V/ma
specification, in addition, having a 2.5VDC offset at vO3 which is the input of the PCM encoder, and also
have a polarity monitor which in this case consists of a comparator circuit with a diode and voltage
divider to perform the needed operation that lets one know whether vp is positive or negative.
Meanwhile, the circuit also has to satisfy the specifications that include input current magnitude being
less than or equal to 10 microamps, no vp contribution at output (perfect common-mode rejection), vO3
and vO4 being in the 0-5V range which is needed for compatibility with the PCM encoder, the voltage
polarity monitor accurate to plus or minus 10mV, and the power supply voltages used should be plus
and minus 15 volts.
Now for the analysis of this design, one has vp simulating the probe current of collected charge, I, which
in turn gets converted to a voltage (vO1) through the initial op amp. The purpose of the difference
amplifier is to create a voltage vO2 with respect to ground that is already scaled to the necessary range
needed for the input of the PCM encoder and the vO2 came out to be 2Vpeak or 4Vpp with the R2 resistors
being 20kohm and the R1 resistors being 10kohm with vp being 1Vpeak smaller than vO1 (when vp is
negative) so one gets an output vO2 through vO2 = (20kohm/10kohm)(vp-vO1) = (20kohm/10kohm)(-1Vpeak)
= -2Vpeak. Now vO1 = vp + I(100kohm) = 10Vpeak + (10microamps)(100kohm) = 11Vpeak and this is why vp-vO1
= -1Vpeak. Now this output vO2 goes into the signal conditioner which is another difference amplifier that
also takes in a DC voltage which creates a DC offset of 2.5V at vO3 and it also takes the signal vO2 and just
scales it by a factor of -1 so all the scaling really was done in the difference amplifier stage. VO3 = (10kohm/10kohm)vO2 + (1+10kohm/10kohm)(V+) = -vO2+(1+1)(10kohm/(110kohm+10kohm))(+15V) = vO2+2.5VDC and this vO3 output ranges from 0.5 V to 4.5V with the 2Vpeak and the +2.5VDC offset. This
satisfies the specifications stated previously. Finally the polarity monitor allows one to determine
whether vp is positive or negative through the use of a comparator with the input at the non-inverting
terminal being vp and the input at the inverting terminal being grounded and since there is no negative
feedback, this op amp is a comparator that passes the +15 VDC power supply voltage to the output when
vp is positive or greater than the 0V of the inverting terminal and it passes -15VDC to the output when vp
is negative or less than the 0V of the inverting terminal. At the output of this comparator, one puts a
diode and at the output of the diode one puts a voltage divider that creates the desired 5V output one
needs. When the diode is on or when it is seeing +15 V, it is turned on and the voltage subsequently
goes through the voltage divider to get a vO4 of = 5V as vO4 = (10kohm/(10kohm+20kohm))(+15V). When
vp is negative or less than the 0V at the inverting terminal of the comparator, -15VDC gets passed and
subsequently the diode is turned off as the turn on voltage is not met and therefore vO4 becomes 0V as it
is grounded. Therefore this voltage polarity monitor works by outputting 5V at vO4 when vp is positive
and 0V when vp is negative.
CIRCUIT DESIGN AND SUPPORTING ANALYSIS
Schematics:
Difference Amplifier Schematic
VO1 = vp + I(100kohm)
VO1 = 10Vpeak + (10microamps)(100kohm)
VO1 = 10Vpeak + 1Vpeak = 11Vpeak
Vp – vO1 = -1Vpeak
VO2 = (20kohm/10kohm)(vp-vO1) = -2Vpeak
Voltage Polarity Monitor Schematic
VO4 = (10kohm/(10kohm+20kohm))(+15V) = +5V when vp > 0V (when it is positive as diode is turned on
and ideally becomes a short circuit with the output of the comparator being +15V when vp is greater
than 0V)
VO4 = 0V (as it is grounded through the voltage divider circuit and as the diode is cutoff or open circuited
ideally as the diode turn on voltage is not met as the output of the comparator is -15V when vp is
negative or less than 0V)
Signal Conditioning Circuitry Schematic
VO3 = -(10kohm/10kohm)vO2 + (1+10kohm/10kohm)(V+)
VO3 = -vO2 + (1+1)[(10kohm/(110kohm+10kohm))(+15V)]
VO3 = -vO2 + 2.5VDC
Circuit Analysis:
Supporting Analysis Equations
(shown in above schematics above)
Discussion of Design Reasoning
What we wanted to do was just perform an offset operation at the signal conditioner so one makes the
scaling at that stage be 1 to 1 so that means vO2=vO3 and so to get vO2 as the output of the difference
amplifier that rejects vp, one picks resistors that give vO2 as 4Vpp or 2Vpeak. In our difference amplifier, our
scaling was performed alongside getting rid of vp. The signal conditioner had the task of adding the 2.5V
offset and one did this through a DC voltage divider at the non-inverting terminal of the difference
amplifier. The polarity monitor works in that when vp is positive, one gets the desired vO4 of +5V while
vO4 is 0V when vp is negative as the polarity monitor is a comparator that passes +15V to its output when
V+>V- and -15V when V->V+. The diode is only turned on and the voltage divider turns the +15V into +5V
when vp is positive as it is compared to the 0V at the inverting terminal of the comparator.
Multisim:
Multisim schematic of all circuitry
Simulated Scope Plots for VO1 (Channel 1)
Simulated Scope Plots for VO2 (Channel 2)
Simulated Scope Plots for VO3(Channel 1)
Simulated Scope Plots for VO4
DATA
Table Comparing Measured and Specified Design Specifications
Measured
Specified
Input Current, I
9.6uA (1V/103.751kohm)
< 10uA
Small-signal transresistance
gain (vo3/I)
VO3 and VO4
226.2 (4.36Vpp /9.6uA =
2.18Vpeak/0.0096mA)
4.36Vpp and 5.76Vpp
200 V/mA
0-5V for both
Scope Plot of VO1(Channel 2)
Scope Plot of VO2(Channel 1)
Scope Plot of VO3
Scope Plot of VO4
DISCUSSION
Percent Error between Measured Values and Theoretical Values
% error = (|theoretical – measured| / |theoretical|)*100%
VO1: %error = (|22.0Vpp - 22.8Vpp| / |22.0Vpp|)*100% = 3.64%
VO2: %error = (|4.0Vpp – 4.12Vpp| / |4.0Vpp|)*100% =3.00%
VO3: %error = (|4.0Vpp – 4.36Vpp| / |4.0Vpp|)*100% =9.00%
VO4: %error = (|5.0Vp – 5.76Vp| / |5.0Vp|)*100% =15.2%
Input current: %error = (|10.0uA – 9.6uA| / |10.0uA|)*100% = 4.00%
Small-signal transresistance gain:%error = (|200.0V/mA – 226.2V/mA|/|200.0V/mA|)*100%=13.1%
Percent Error between Measured Values and Simulated Values
% error = (|simulated – measured| / |simulated|)*100%
VO1: %error = (|21.9Vpp - 22.8Vpp| / |21.9Vpp|)*100% = 4.11%
VO2: %error = (|4.0Vpp – 4.12Vpp| / |4.0Vpp|)*100% =3.00%
VO3: %error = (|4.0Vpp – 4.36Vpp| / |4.0Vpp|)*100% =9.00%
VO4: %error = (|5.39Vp – 5.76Vp| / |5.39Vp|)*100% =6.86%
Discussion of the Significance of Your Results
In this experiment, our group was not able to get a decent oscilloscope capture of our vO4 output voltage
waveform as we experience difficulties with our circuit. We tried debugging the circuit but everything
seemed to be correct. We had put in bypass capacitors in place and that fixed some of the problems we
were having with the noise in the circuit but we were not able to get the results that we wanted for the
vO4 output voltage. Also in our Multisim capture of the vO4 voltage had errors that cannot be explained
as one could not get the expected 5V output voltage as one also could not explain why this is. As a result
of inaccurate VO4 measurements, our percent error calculations based off of these values were affected
greatly.
The percent errors for the measured and theoretical values ranged from as low as 3.64% for VO1 to as
high as 15.2% for the flawed vO4. The percent errors for the measured and Multisim simulated values
ranged from as low as 3.00% for VO2 to as high as 9.00% for VO3. When comparing the two percent error
categories, one can see that they are basically the same for vO2 and vO3 as one would expect due to the
theoretical and Multisim values of the ac component which is vO2 and vO3 of the output voltage being
exactly the same at 4.00V and as the measured values are the same ones used in both cases. Overall,
the design did a very effective job of completing its task and the actual measurements of the voltages
came very close to its theoretical and Multisim simulated values based on the project design.